This cheevo will be awarded for playing a FULL level in coop.
Now this is possible to do by your self but if some1 is willing to help its more fun if there are two of you.
The second player does not have to have a profile assigned to it but it has been confimed by Sun Rei Shin that if you do sign in a 2nd profile both of you will get the cheevo
Now the 2nd player MUST be on split screen local coop and MUST be active before the level starts, if you start a level then start up the 2nd player you will not get the cheevo.
UPDATE 16/05/11: Just confirmed by MonkyPorter1983
that this can be achieved in Free Play mode.